Speech enhancement (SE) frontends for ASR that address background noise have therefore been widely studied [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}. Single-channel SE is a general and challenging setting that does not require any special device such as a microphone array. Early DNN-based ASR studies showed single-channel SE improves the ASR performance under noisy environments [1]}, [2]}, [3]}. Whereas, recent work revealed that although single-channel SE has successfully improved SE evaluation metrics, e.g., signal-to-noise ratio (SNR), this improvement does not necessarily lead to better ASR performance, when using state of the art ASR backends, e.g., lattice-free version of the maximum mutual information [4]}, and end-to-end models [5]}.
